1. Field of the Invention
This invention relates to a micellar slug suitable for use in a micellar drive for recovering oil from subterranean reservoirs. More specifically, it relates to a mobility controlled micellar slug for oil recovery having a high capability for maintaining micro-emulsions against the change of the composition of the micro-emulsion, since micro-emulsions can be formed at a high salt concentration and in a wide composition range. This invention also relates to a process for producing or recovering oil from an oil-bearing subterranean reservoir at a high recovery efficiency.
2. Description of the Prior Art
It is well known in the art that the so-called "primary recovery" methods, including pumping methods, can recover only a portion of the petroleum or crude oil (referred to as "oil" hereinafter) from subterranean reservoirs and that they leave substantial amounts of oil in the subterranean reservoirs.
In order to recover the remaining large amounts of oil from the subterranean reservoirs, various so-called "enhanced oil recovery" (EOR) methods have been proposed.
Of the EOR methods, the recent "micellar drive" methods are to be noted. According to these methods, a micellar slug, that is, a clear micro-emulsion derived from water and oil such as petroleum, petroleum distillates, or fuel oil, is injected under pressure into the subterranean reservoirs for the recovery of oil in the subterranean reservoirs. These EOR methods are disclosed in, for example, U.S. Pat. Nos. 3,506,070, 3,613,786, 3,740,343, 3,983,940, 3,990,515, 4,017,405, and 4,059,154. These prior arts disclose that various kinds of surfactants, including anionic-, nonionic-, and cationic-type surfactants, can be used alone or in any mixture thereof in the formation of the micellar slugs. Examples of such surfactants are petroleum sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates, alkane sulfonates, polyoxyethylene alkylether sulfates, polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers, polyol fatty acid esters, alkyltrimethyl ammonium salts, and dialkyldimethyl ammonium salts.
The surfactants used in the formation of micellar slugs must be available at a low cost, since a large amount of micellar slugs is used for the recovery of oil from subterranean reservoirs, and the surfactants also must fulfill the above-mentioned requirements. Furthermore, the surfactants used in micellar slugs should have good thermal stability, salinity tolerance, and hard-water resistance because, since numerous oil production wells or oilfields are present in the world, subterranean reservoirs have variety of properties and also a variety of available water, for example, from soft water containing no substantial amount of inorganic salts to brine containing large amounts of inorganic salts and polyvalent metallic ions.
It is known in the art that petroleum sulfonate is an optimum surfactant usable as an injection fluid in a micellar drive. However, a problem exists in that petroleum sulfonate has an unsatisfactory salinity tolerance and hard-water resistance and, therefore, can be applied only in the case of special oilfields. For this reason, various attempts have been made to improve the properties of petroleum sulfonate by using petroleum sulfonate together with other surfactants or water-soluble polymer-thickening agents. However, micellar slugs or surfactants having the desired properties and a satisfactory cost have not been obtained.
The inventors have proposed the use of alpha-olefin sulfonates as a surfactant in micellar slugs which have an excellent capability for decreasing interfacial tensions comparable to or better than those of petroleum sulfonates and have an excellent salinity tolerance and hard-water resistance and a relatively high viscosity. However, problems still exist in that micro-emulsions are readily broken when the micro-emulsions are injected into oil-bearing subterranean reservoirs, since the composition range thereof in which micro-emulsions can be formed is not wide, and that the production of micellar slugs having a low viscosity is not easy.
Furthermore, the viscosity of micellar slugs is important in the practice of the micellar drive methods. That is, an oil recovery efficiency may be increased by mobility control i.e., adjusting the viscosity of micellar slugs to substantially equal or little higher than that of the oil remaining in the subterranean reservoirs. No increase in an oil recovery efficiency cannot be obtained without mobility control.